1. Field of the Invention
The invention relates in general to energy storage systems for use in AC distribution networks and in particular to a hybrid energy storage system including different types of power conversion devices to reduce power consumption.
2. Prior Art
Energy storage systems are used in power distribution systems to provide power to loads when the loads are disconnected from the host power grid. A conventional energy storage system is a battery energy storage system (BESS). FIG. 1 is a schematic diagram of a portion of an AC power distribution system including a BESS shown generally at 10. The power distribution system includes a utility bus 12 for providing electricity to a load 14 through a switch 16 (e.g. circuit breaker). If disturbances occur on the host utility bus 12, the switch 16 is opened and the load 14 is disconnected from the utility bus 12.
The BESS 10 includes a transformer 18 for coupling the AC power from utility bus 12 to a power converter 20. Power converter 20 converts bi-directionally from AC to DC and DC to AC. Battery 22 stores the energy imported from utility bus 12 and converted from AC to DC by the power converter 20. If switch 16 is opened due to a disturbance on the utility bus 12, DC power from the battery 22 is converted to AC through power converter 20 and supplied to load 14 through transformer 18. Inductor 24 and capacitor 26 filter unwanted components (e.g. harmonic currents).
When connected to the utility bus 12, the BESS 10 supplies reactive power which maintains the terminal voltage. In addition, the frequency and phase of BESS voltage is adjusted to supply the required active power. When switch 16 is opened, isolating the BESS 10 from the utility bus 12, the power converter 20 establishes the required frequency and voltage. A disadvantage of the conventional BESS is that the load 14 is subject to the disturbances on the utility bus 12 for a short period time until the disturbance is removed, or the switch 16 is opened to isolate the load 14 and the BESS 10 from the disturbance. In some applications, a portion of the load 14 is very sensitive and is unacceptably affected during the period of time until the disturbance is removed or isolated.
FIG. 2 is a schematic diagram of a BESS 10 coupled to a critical portion of a load. Switches 32 and 34 connect the critical load 38 and the balance of the load 40 to the AC system 30. The power consumption and quality is monitored at metering point 36. When the BESS 10 is connected to the AC system 30, it provides various power control functions. As described above, the BESS 10 regulates voltage to the critical load 38. The BESS 10 also moderates the effects of load swings and impact loads, improves power system stability for remote or weak utility connections, combines active and reactive power to provide voltage support and provides energy management and standby power benefits.
FIG. 3 is a schematic diagram of the system in FIG. 2 after a disturbance has occurred in the AC system 30 and switches 32 and 34 are opened. In this mode of operation, the BESS 10 provides power to the critical load 38. In order to reach the condition shown in FIG. 3, the switch 34 must quickly isolate the BESS 10 and the critical load 38 from the AC system 30 during the disturbance (particularly voltage depression or momentary outage). During isolated operation, the BESS 10 must regulate frequency through active power control (a governor function) and regulate voltage through continuous control of reactive power (a voltage regulator function). When isolated, the BESS 10 provides necessary active and reactive power and energy for these regulation functions.
FIG. 4 is a schematic diagram of another conventional system for providing power to a critical or protected load. As shown in FIG. 4, the AC system 30 is connected to a protected load 54 through a switch 52 and an uninteruptable power supply (UPS) 50. The UPS 50 includes two power convertors 56 and 58 and battery 60. Power converter 56 converts the AC power from the AC system 30 to DC power and charges battery 60. Power converter 58 converts DC power, from either power converter 56 or battery 60, into AC power and provides the AC power to protected load 54.
Unlike BESS 10, UPS 50 is positioned between the protected load 54 and the AC system 30. Consequently, when there is a disturbance on the host grid, the protected load 54 is effectively instantaneously isolated, and therefore protected, from the upset. The disadvantage of this approach is that all of the power provided to the protected load 54 must pass through the UPS 50 on a continuous basis. The converters 56 and 58 each must have a power rating at least equal to that of the protected load 54. There are significant electric losses incurred in the two converters 56 and 58. The losses and equipment ratings tend to limit the practical application of the UPS 50 to relatively small loads, or to large sensitive homogeneous loads for which substantial penalties result from disturbances.
A summary of the BESS and UPS is provided in Table I below.
TABLE I ______________________________________ FEATURE BESS UPS ______________________________________ connection parallel connected series connected isolation requires switching to isolate dc bus buffers disturbances bus (speed dictated by application) losses negligible when not losses due to load current charging or discharging passing through converters (if no bypass switch) power one: acdc two: ac.fwdarw.dc and dc.fwdarw.ac converters voltage continuous not normally possible regulation power flow bi-directional flow possible unidirectional for both real and reactive power ______________________________________
In practice, many loads (e.g., a factory, an office building, a mine, etc.) are composed of a heterogeneous aggregation of smaller loads which exhibit varying degrees of sensitivity to disruptions. In such a heterogeneous load, there are likely to be components which are very sensitive and other components which can tolerate the effects of a disturbance until it is removed or isolated. The use of a UPS to protect the entire load maybe uneconomic, and the use of a BESS will not meet the functional requirements of the very sensitive portion of the load. Therefore, there are limits to the use of BESS and UPS alone.